The entire disclosures of Japanese Patent Application No. 2001-080955 filed on Mar. 21, 2001 and Japanese Patent Application No. 2001-177192 filed on Jun. 12, 2001 including specifications, claims and summaries are incorporated herein by reference in their entireties.
Not Applicable
Not Applicable
1. Field of the Invention
The present invention relates to a recording medium, particularly to a recording medium suitable for an ink a jet printer.
2. Discussion of the Background
An ink jet recording system is a system wherein ink droplets are jetted at a high velocity from a nozzle to form an image directly on a recording medium. A printer employing such an ink jet system has found remarkable widespread use in recent years, since it can easily be small-sized, it is easy for full coloring or high speed modification, or its printing noise is low.
As a recording medium for an ink jet printer, one having a porous ink receiving layer comprising fine inorganic particles such as silica or alumina and a binder such as polyvinyl alcohol, formed on a substrate such as a paper or a film in order to quickly absorb ink and to obtain a clear image, has been known. The recording medium for an ink jet printer is required to absorb the solvent contained in a large quantity in the ink by pores in the ink receiving layer, and accordingly, the ink receiving layer is required to have pores with a large pore radius and a large pore volume. Further, as the ink receiving layer is more transparent, a clearer image having a high color density can be formed, and accordingly, the ink receiving layer is preferably one having good transparency.
Further, in addition to the above-mentioned ink absorptivity and transparency, it has become important that, as an aqueous ink is used in an ink jet recording system, even when a recorded product is in contact with water, no running of ink should take place as a result of flowing of a dye in the ink (hereinafter referred to as water resistance), or even if the surface of the recording medium is in contact with a hard object, it should be free from receiving scratches and thus free from impairment of the quality of the recorded product (hereinafter referred to as scratch resistance), or the surface gloss is high (hereinafter referred to as glossiness).
In order to cope with these requirements, many ink jet recording media have heretofore been proposed. JP-A-2000-21892 discloses a recording sheet having a high color density and gloss, which comprises a substrate, a porous layer containing boehmite, formed on the substrate, and a porous layer having composite particles comprising silica and alumina bound by a binder, formed on the porous layer containing boehmite. The process for producing such a recording sheet is a process wherein a coating fluid composed of a composite sol comprising silica and alumina, is coated and then dried as pressed against a die having a smooth surface. Accordingly, such a process is applicable to a case where a paper is used as the substrate as in Examples given in the publication, but it can not be applied in the case of a water-impermeable substrate such as a resin film or a resin coated paper having a polyolefin resin coating layer (so-called RC paper), since the solvent in the coating fluid can not be thereby evaporated and removed at the time of drying.
Further, JP-A-2000-351267 discloses a recording medium for pigment ink, which comprises a substrate, an ink receiving layer containing boehmite, formed on the substrate, and a layer formed from a coating fluid containing oxide particles such as alumina ultra fine particles or silica particles treated with aluminum polychloride, having an average particle size of from 10 to 200 nm, and having a pH of from 3 to 11, laminated on the ink receiving layer.
It is an object of the present invention to provide an ink jet recording medium which has ink absorptivity suitable for ink jet recording and which is suitable for recording in high color density and further is excellent in water resistance, scratch resistance and glossiness.
The present invention provides an ink jet recording medium comprising a substrate, an ink receiving layer containing fine inorganic particles, formed on the substrate and a layer containing silica/alumina composite particles, formed on the ink receiving layer, wherein the layer containing silica/alumina composite particles, is a layer containing a xerogel having an average pore radius of at least 6.0 nm, obtained by removing a solvent from a silica/alumina composite sol containing agglomerated particles comprising silica and alumina.
The layer containing silica/alumina composite particles, is a layer obtained by removing a solvent from a silica/alumina composite sol containing agglomerated particles comprising silica and alumina as colloidal particles. The silica or alumina may be hydrate of silicon oxide or hydrate of aluminum oxide respectively. The coating fluid obtained by mixing the silica/alumina composite sol, a binder and a solvent, preferably forms a porous layer having the silica/alumina composite particles bound by the binder (hereinafter referred to as a composite particle layer).
In the present invention, the xerogel used for the composite particle layer is required to have an average pore radius of at least 6.0 nm. The xerogel is obtained by removing the solvent from the silica/alumina composite sol. The pore characteristics are measured by a nitrogen absorption desorption method. The average pore radius is a value obtained by calculation by (2V/Axc3x97103 (nm), where V is the total pore volume (cm3/g) and A is the specific surface area (m2/g). If the average pore radius of the xerogel obtained by removing the solvent from the silica/alumina composite sol, is less than 6.0 nm, the ink absorptivity of the composite particle layer tends to be inadequate, such being undesirable. The average pore radius of the xerogel is preferably within a range of from 6.0 to 15 nm, more preferably from 6.5 to 12 nm, particularly preferably from 7.0 to 10 nm.
The specific surface area of the xerogel is preferably from 50 to 200 m2/g. If the specific surface area is smaller than 50 m2/g, not only the fixing property of the dye in the ink tends to be poor, but also the glossiness and the transparency of the composite particle layer tend to be poor, and it tends to be difficult to obtain a recording medium having a high color density and glossiness. Further, if the specific surface area exceeds 200 m2/g, it tends to be difficult to obtain a large average pore radius, and it tends to be difficult to obtain a recording medium having good ink absorptivity. A more preferred range of the specific surface area is from 60 to 140 m2/g. By adopting the specific surface area within such a specific range, it is possible to obtain a recording medium which is excellent in glossiness and color density and which is excellent also in ink absorptivity.
In the present invention, the composite particle layer is formed on the ink receiving layer, whereby a recording medium excellent in ink absorptivity, image color density, water resistance, scratch resistance and glossiness, can be obtained. In particular, it is possible to obtain a recording medium of a high quality having a high color density and high glossiness, which is free from beading in a printing test which will be described hereinafter.
Not Applicable
Now, the present invention will be described in detail with reference to the preferred embodiments.
In the present invention, the silica/alumina composite sol is preferably a colloidal solution obtained by adding to a silica sol an aluminum salt which shows acidity when dissolved in water, wherein the average particle size of agglomerated particles is from 50 to 200 nm. If the average particle size of the agglomerated particles exceeds 200 nm, the transparency of the composite particle layer tends to decrease, and the color density of an image tends to be low, such being undesirable. On the other hand, if it is smaller than 50 nm, although the transparency is good, the average pore radius tends to be small, and the ink absorptivity tends to be poor, such being undesirable. When the agglomerated particle size is within this range, the average pore radius when formed into the xerogel, can be made large, and it is possible to form a composite article layer which satisfies both the ink absorptivity and the transparency.
The silica in the agglomerated particles in the silica/alumina composite sol is preferably such that the primary particles are spherical, and the average particle size of the primary particles is from 20 to 70 nm. The recording medium of the present invention has high scratch resistance, since the primary particles of silica in the silica/alumina composite sol are spherical. If the average particle size of the primary particles of silica is smaller than 20 nm, when the silica/alumina composite sol is dried, it tends to be difficult to obtain a xerogel having a large average pore radius, and the ink absorptivity of the composite particle layer tends to be inadequate, such being undesirable. On the other hand, if the average particle size of the primary particles of silica exceeds 70 nm, the specific surface area of the silica/alumina composite particles tends to be small, whereby not only the fixing property for a dye tends to be inadequate, but also the glossiness and transparency of the composite particle layer tend to be poor, and it tends to be difficult to obtain a recording medium having high color density and glossiness, such being undesirable. A more preferred range of the average particle size of the primary particles of silica is from 20 to 60 nm. By adjusting the average particle size of silica in the silica sol to be used as the raw material within such a specific range, the specific surface area of the xerogel obtainable by drying the silica/alumina composite sol can be brought within the above-mentioned specific range, and it is possible to obtain a recording medium which is excellent in glossiness and color density and which is excellent also in ink absorptivity. The average particle size of the primary particles of silica is measured by a transmission electron microscope.
The silica/alumina composite sol preferably has a pH of from 3 to 9. If the pH is higher than 9, the zeta potential of the agglomerated particles tends to be low, such being undesirable. On the other hand, if the pH is lower than 3, the alumina tends to be dissolved, such being undesirable. The silica/alumina composite sol preferably has a zeta potential of agglomerated particles of +10 mV or higher, whereby the fixing property for an anionic dye to be used for e.g. an ink jet printer, will be high. A more preferred range of the zeta potential is from +30 to +90 mV.
With the silica/alumina composite sol, as the amount of alumina increases relative to silica, the zeta potential of agglomerated particles tends to be high. The amount of alumina is preferably an amount whereby the zeta potential of agglomerated particles becomes +10 mV or higher. To the silica sol as the raw material, it is necessary to add alumina in a larger amount, as the specific surface area of the xerogel obtainable by removing the solvent, is larger. It is preferred to add at least 1 g as Al2O3 per 100 g of the SiO2 component in the silica sol.
With respect to the coated amount of the composite particle layer, it is preferred that the total amount of the silica/alumina composite particles and the binder after drying per unit area is preferably from 0.1 to 10 g/m2. If the coated amount is less than 0.1 g/m2, no adequate image color density, water resistance, scratch resistance or glossiness tends to be obtainable, such being undesirable. On the other hand, if the coated amount exceeds 10 g/m2, the strength of the composite particle layer tends to deteriorate, such being undesirable.
The recording medium of the present invention has an ink receiving layer containing fine inorganic particles (hereinafter referred to as a lower layer to distinguish it from the composite particle layer) beneath the composite particle layer. As the fine inorganic particles in the lower layer, it is preferred to employ alumina hydrate, alumina or silica, whereby a porous layer having a large pore volume can be formed, and the ink absorptivity is excellent.
To form such a lower layer, a coating fluid comprising the fine inorganic particles, a binder and a solvent, is coated on a substrate, followed by drying to form a porous lower layer. It is preferred to form a porous layer containing alumina as the lower layer, whereby not only the ink absorptivity but also the fixing property for a dye will be excellent. Further, alumina hydrate such as boehmite is more preferred, since it is excellent not only in the ink absorptivity and the fixing property for a dye but also in transparency, and recording with a high color density can be attained. As a specific example of the alumina hydrate such as boehmite, an alumina sol or the like disclosed in JP-A-10-231120, may be mentioned.
Further, it is preferred to use silica as the fine inorganic particles for the lower layer, whereby a porous layer having a large pore volume can be formed, and the ink absorptivity is excellent. The fine silica particles are not particularly limited, and wet-process silica or dry-process silica may suitably be employed. Among them, dry-process silica having a primary particle size of at most 30 nm, is particularly preferred, since the primary particle size is small, and it is excellent in dispersibility in water and capable of forming a porous layer excellent in smoothness.
However, since the surface is negatively charged, the fine silica particles do not provide a fixing property for an anionic dye which is commonly used in a dye ink for an ink jet printer, and the water resistance of an image will be poor. Therefore, when fine silica particles are employed as the fine inorganic particles, it is preferred to incorporate a cationic compound such as a cationic polymer. The cationic polymer to be mixed with fine silica particles, is not particularly limited, and a polymer containing a quaternary ammonium salt, may, for example, be mentioned.
With respect to the coated amount of the lower layer, the total amount of the fine inorganic particles and the binder after drying is preferably from 5 to 100 g/m2 of the substrate, from the viewpoint of the ink absorptivity. If the coated amount is less than 5 g/m2, the ink absorptivity tends to be inadequate, such being undesirable. On the other hand, if the coated amount exceeds 100 g/m2, no further improvement in the ink absorptivity will be observed, and not only the mechanical strength tends to deteriorate, but also the material will be wasted, such being undesirable.
The substrate is not particularly limited, and various types may be employed. In addition to papers made mainly of cellulose, synthetic papers, non-woven fabrics, etc., various water-impermeable plastic films of e.g. a polyester resin such as polyethylene terephthalate, a polycarbonate resin, a fluororesin or a polyvinyl chloride resin, and resin-coated papers (hereinafter referred to as RC papers) having a polyolefin resin coating layer on the surface, may, for example, be mentioned. For the recording medium of the present invention, as the substrate, it is preferred to employ a water-impermeable substrate.
Among them, a polyethylene terephthalate film is preferred. Particularly preferred is a white colored polyethylene terephthalate film having a white pigment incorporated, since it is excellent in the surface smoothness, glossiness and durability, and an ink jet film of a high quality can thereby be obtained. Further, a RC paper is also particularly preferred, since it is excellent in the surface smoothness and glossiness, and an ink jet paper having a texture similar to a photographic paper can thereby be obtained.
For both the composite particle layer and the lower layer, the method for coating the coating fluid is not particularly limited, and a method of using a bar coater, a die coater, a gravure coater, an air knife coater, a blade coater, a comma coater, a slide hopper or a curtain coater, may, for example, be mentioned.
For both the composite particle layer and the lower layer, the binder for the coating fluid is not particularly limited, and an organic substance, such as polyvinyl alcohol or its modified product, starch or its modified product, SBR latex, NBR latex, hydroxycellulose, or polyvinyl pyrrolidone, may be employed. In a case where polyvinyl alcohol is employed, it is preferred to add boric acid or a borate such as borax, as a crosslinking agent, as the case requires, whereby the strength of the coated layer may be increased, and cracking of the surface or the like may be prevented.
A method for forming the lower layer and the composite particle layer on the water-impermeable substrate, is not particularly limited. The coating fluid for the lower layer may be coated on the substrate and then dried, whereupon the coating fluid for the composite particle layer may be coated and again dried. Otherwise, the coating fluid for the lower layer and the coating fluid for the composite particle layer may be coated simultaneously on the substrate, and the two layers may simultaneously be dried.
However, in a case where the substrate has low heat resistance, like RC paper, drying can not be carried out at a high temperature. Accordingly, the coating fluid for the lower layer and the coating fluid for the composite particle layer may be coated simultaneously on the substrate and then cooled to immobilize the coated layer by gelation, followed by drying by dry air at a temperature of not higher than 70xc2x0 C. In order to impart such a nature of gelation upon cooling to the coating fluid, it is necessary to optimize the solid content concentration in the coating fluid or to optionally add a crosslinking agent such as boric acid or borax, depending upon the fine inorganic particles and the binder to be used.
Further, to the coating fluid, an additive to improve ozone resistance or light resistance of an image, may be incorporated, as the case requires.
Now, a method for producing the silica/alumina composite sol will be described. The pH or the solvent for the silica sol as the raw material for the silica/alumina composite sol, are not particularly limited. However, with respect to the solvent, water is preferred from the viewpoint of the simplicity in operation. For example, it is preferred to use a silica sol commercially available such as one known by a trade name Cataloid SI-50, manufactured by Catalysts and Chemicals Industries Co., Ltd. The silica sol may be diluted with water.
As the aluminum salt whereby the solution becomes acidic when dissolved in water, a salt of aluminum hydroxide with a strong acid (hereinafter referred to simply as an acidic aluminum salt) is preferred. The acidic aluminum salt may, for example, be an inorganic salt such as aluminum chloride, aluminum sulfate or aluminum nitrate, or an organic salt such as aluminum acetate. It is preferred that such an acidic aluminum salt is suitably dissolved in water and mixed to the silica sol.
As the acidic aluminum salt, aluminum polychloride is preferred, the aluminum polychloride is a compound represented by the chemical formula [Al2(OH)nCl6xe2x88x92n]m(1 less than n less than 6, m less than 10). For example, one commercially available by a trade name such as Takibine #1500 or PAC250A, manufactured by Taki Chemical Co., Ltd., may be mentioned. The aluminum polychloride preferably has a basicity of at least 20%. The basicity is represented by (n/6) in the above-mentioned formula by percentage, and the specific method of measurement is defined by JIS K1475. If the basicity is smaller than 20%, the content of Cl is large relative to Al, such being undesirable when impurity elements are to be removed by e.g. ultrafiltration.
As a method for adding the acidic aluminum salt to the silica sol, it is preferred that a predetermined amount of the acidic aluminum salt is gradually added to the silica sol as the raw material. As the acidic aluminum salt is gradually added to the silica sol, alumina will gradually form and deposit on the surface of silica particles in the sol. As the deposited amount of alumina increases, the surface potential of the sol particles changes from negative to positive. On the way, the potential passes through a state of 0, whereby agglomeration of particles takes place to form agglomerated particles comprising silica and alumina. At the time of adding the acidic aluminum salt, it is preferred to stir the silica sol to prevent local concentration of the acidic aluminum salt. Inversely, if the silica sol as the raw material is gradually added to the solution of the acidic aluminum salt, a sol containing complex particles having alumina deposited on the surface of silica sol particles, may be formed, but agglomerated particles will not substantially be formed. Accordingly, the xerogel obtainable by drying the sol will be one having a small average pore radius. Thus, if an ink receiving layer is formed by using such a sol, the ink absorptivity will be poor, and the fixing property for a dye will be inadequate.
The temperature at the time of mixing the silica sol and the acidic aluminum salt is preferably from 25 to 150xc2x0 C. If the temperature is lower than 25xc2x0 C., the reaction speed tends to be slow, and alumina may not be sufficiently deposited on the surface of silica particles, such being undesirable. If the temperature is higher than 150xc2x0 C., the operation tends to be difficult.
The amount of the acidic aluminum salt to be added, is preferably an amount whereby the zeta potential of particles will be +10 mV or higher. It is necessary to add the acidic aluminum salt in a larger amount, as the specific surface area of the sol particles in the silica sol as the raw material is larger. However, in the case of a silica sol to be used as the raw material wherein the average particle size of primary particles is from 20 to 70 nm, it is preferred to add the acidic aluminum salt in an amount of from 1 to 50 g as calculated as Al2O3, per 100 g of silica as calculated as SiO2.
Even if the amount of the acidic aluminum salt is excessive, there is no particular problem with respect to the properties of the silica/alumina composite sol thereby obtained. However, the after mentioned operation for removing impurity elements by e.g. ultrafiltration, tends to be difficult, such being disadvantageous.
To the silica sol, another electrolyte may further be incorporated in addition to the acidic aluminum salt, whereby agglomerated particles may be formed more effectively. Such another electrolyte is not particularly limited so long as it has an agglomerating action to the silica sol. For example, sodium chloride, potassium chloride, sodium sulfate, potassium acetate or magnesium nitrate may be mentioned. These electrolytes may be used alone or in combination as a mixture.
The amount of such another electrolyte is preferably from 1 to 70 wt %, based on the weight of silica (calculated as SiO2) in the silica sol as the raw material. The method of adding such an electrolyte is not particularly limited, and such an electrolyte may be preliminarily added to the silica sol, or it may be added to the acidic aluminum salt, and then added to the silica sol. Otherwise, the electrolyte may be added to a mixed solution obtained by adding the acidic aluminum salt to the silica sol.
Then, from the mixed solution after adding the acidic aluminum salt to the silica sol, it is preferred to remove impurity ions such as an unreacted acidic aluminum salt or the added electrolyte. To remove such impurity ions effectively, it is preferred to adjust the pH to from 5 to 10, more preferably from 6 to 8, by adding an alkali such as sodium hydroxide or an acid such as hydrochloric acid, to the mixed solution after adding the acidic aluminum salt to the silica sol. As the method for removal, ultrafiltration is preferred.
In a case where the average particle size of the agglomerated particles of the silica/alumina composite sol synthesized as described above, is larger than 200 nm, it is adjusted to a level of from 50 to 200 nm by adding a peptitizer or by carrying out ultrasonic dispersion. Peptitizer is not particularly limited, and an inorganic acid such as hydrochloric acid, nitric acid, sulfuric acid or amide sulfuric acid, or an organic acid such as acetic acid, may suitably be used. These peptitizers may be used alone or in combination as a mixture.